This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This description is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Corrosion is defined as a chemical or electrochemical reaction between a material, usually a metal, and the environment that deteriorates the properties of the material. Metallic corrosion cost the industries of the United States an estimated $170 billion annually. Various industries that are affected by the detrimental effects of corrosion include electrical power plants, chemical processing plants, oil/gas production and refineries, water and wastewater management, among others.
In the oil and gas industry, the iron (Fe) in a steel pipe has a tendency to corrode in the presence of corrosive materials that are by-products of the hydrocarbon production, including oxygen (O2), hydrogen sulfide (H2S), and carbon dioxide (CO2). The corrosion process releases Fe2+ ions and electrons which reduce the corrosive materials. The released Fe2+ ions react with the products of the reduction to form corrosion by-products, such as iron(II) hydroxide (FeOH2), iron sulfide (Fe2S3), or iron carbonate (Fe2CO3), among others, within the flow stream of the oil and gas.
Corrosion can be enhanced by the aqueous fluid that is inevitably produced alongside hydrocarbons during the production of crude oil and natural gas. Within the aqueous fluid, the natural occurrence of corroding agents alone, such as carbon dioxide (CO2) and hydrogen sulfide (H2S), can lead to significant corrosion problems. Additionally, the CO2 and H2S can combine with water to form carbonic acid (H2CO3) and dissolved hydrogen sulfide (H2S), respectively. The formation of such acids further increases the rate of corrosion. For example, the formation of H2CO3 can significantly lower the pH of water and increase corrosion formation resulting in pitting corrosion and possibly the formation of hairline cracks throughout the production system.
There are numerous types of corrosion which are usually classified by the cause of the material deterioration. Galvanic corrosion is a type of corrosion that can occur when metals or semi-metals having varying electrode potentials come into contact with each other through the use of an electrolytic material such as water. The electrolytic material provides a means for ion migration whereby ions of a less noble metal gravitates to a more noble metal. This movement causes the less noble or less stable metal to corrode more rapidly. FIG. 1 is a galvanic corrosion chart 100. The chart 100 contains the galvanic series ranks of metals and semi-metals according to their potential and determines the nobility of such materials. Metals that are less noble, or anodic, and that will corrode more easily are contained at the negative end 102 of the chart 100. Conversely, metals that are more noble, or cathodic, and that are more resistant to corrosion are contained at the positive end 104 of the chart 100. During galvanic corrosion, the anode metal will sacrifice its electrons resulting in decomposition. The cathode metal accepts the released electrons and is protected from corrosion. It should be noted that chart 100 is drawn up for metals and semi-metals in seawater. Therefore, while the relative position of the metals on chart 100 may change in other environments, it is the distance between the metals on chart 100 that indicates the risk for galvanic corrosion. Although galvanic corrosion may occur more quickly when metals of different type are in electrical contact, it will still occur in neat metals, due to the presence of more electropositive and electronegative sites.
An example of this is pitting corrosion. Pitting corrosion, or pitting, is a form of localized galvanic corrosion that leads to the creation of small holes in the metal. The driving power for pitting is the depassivation of a small area, which becomes anodic while an unknown, but possibly large area, becomes cathodic, which can lead to very localized galvanic corrosion. Pitting can be initiated by localized chemical or mechanical damage to a protective oxide film or to the metal, low dissolved oxygen concentrations, or high concentrations of contaminants in source water. Additionally, crevice corrosion is a form of localized pitting which takes place in narrow clearances or cervices on a surface of a metal where fluid has become stagnant.
Since preventing corrosion may be difficult in certain environments, one of the most economical solutions is to control the corrosion rate. There are various methods used to slow corrosion including chemical inhibition, coatings, or corrosion resistant alloys. Each of these methods has its own advantages and disadvantages with the cost to implement the method usually dictating which particular method to use.
Chemical inhibitors, such as neutralizers, film forming reagents, and non-nitrogen-based corrosion inhibitors, may be utilized to provide protection to a surface in contact with a flowing stream. The chemical inhibitor may be added to the flow stream and thereby deposits a thin film upon a surface of the system. The thin film facilitates the prevention of various reactions between corrosive compounds in the flow stream and that particular surface. Likewise, coating inhibitors may be painted or sprayed onto a surface to act as a barrier to inhibit contact between corrosive materials and the surface. Corrosion resistant alloys may also be used, including mixtures of various metals such as chrome, nickel, iron, copper, and cobalt, among others. Such metals in combination provide corrosion resistance more effectively than a surface composed of only one type of metal.